The present invention relates to an autotensioner for keeping the tension of a power transmission member such as a toothed belt or a chain to a predetermined level.
Since a power transmission member in the form of a belt mounted on an internal combustion engine is subject to change of tension with the fluctuation of the center-to-center distance between pulleys owing to a thermal expansion of the engine during operation or with the fluctuation of torque, it is a common practice to provide an autotensioner to apply tension to the belt through a tension roller and thus to keep constant the tension of the belt.
One such prior art autotensioner comprises a cylinder filled with a hydraulic oil, a piston slidably mounted in the cylinder and a pressure control spring for biasing a rod of the piston in such a direction as to protrude from the cylinder (Japanese Unexamined Utility Model Publication 62-40355).
With this type of prior art autotensioner, it is necessary to introduce hydraulic oil into the cylinder under vacuum so that it can replace the air in the cylinder. Thus it is extremely troublesome and time-consuming to assemble such an autotensioner.
Further it is necessary to seal the hydraulic oil in the cylinder by use of a diaphragm seal having a complicated configuration to absorb fluctuations in the amount of oil resulting from relative movements of the cylinder and the piston and fluctuations in the volume of oil owing to heat. Such a seal tends to be costly.
An autotensioner of an oil-sealed type can be theoretically oriented in any desired direction. For all practical purpose though, it is mounted in such a direction that its rod extends vertically in most cases in view of the layout of engine and the like. Thus, the advantage of the oil-sealed type is not fully utilized.
It is an object of the present invention to provide an autotensioner which obviates the abovesaid shortcomings and which is reliable and inexpensive.